1. Field of the Invention
The present invention relates to a cleaning method and a cleaning apparatus of a semiconductor surface such as silicon surface, in which a residual water stain, the so-called water mark can be reduced in a step of drying silicon after wet cleaning or etching of a silicon surface.
2. Description of the Related Art
In the semiconductor industry using silicon, the material of silicon is classified into single crystal silicon, polycrystalline silicon, and amorphous silicon in view of the configuration. Silicon in any configuration has characteristics as a semiconductor, and plenty of raw material exists on earth. Thus, silicon is used in a wide field. The single crystal silicon is used for a memory such as a DRAM and EPROM or an arithmetic unit such as a CPU and MPU. The polycrystalline silicon is used for a switching transistor and a driving circuit of a liquid crystal display device, or a solar cell. The amorphous silicon is used for a switching transistor of a liquid crystal display device or a solar cell.
Among applications of each silicon, silicon is often used particularly for an integrated circuit using the function as a transistor. Especially, in the case where silicon is used for an integrated circuit, research and development competition are keen to make a clean environment or a micro machining apparatus and to develop material techniques, for the sensitivity of silicon to impurities and minute machining technique.
The technique of cleaning and etching is indispensable when silicon is used as a semiconductor. Cleaning is carried out in a wet mode in most cases. As the cleaning for removing physically adsorbed materials, there are scrub cleaning in which cleaning is carried out by scrubbing the surface by a brush or the like, ultrasonic cleaning or megasonic cleaning in which cleaning is carried out by the impact of compressed waves generated in pure water or a solution by ultrasonic waves, or the like. As the cleaning for removing organic materials attached to the surface, there is cleaning in which silicon is immersed in oxygenated water (hydrogen peroxide water) mixed with sulfuric acid to remove the organic materials by a chemical reaction, or the like. As the cleaning for removing metal contaminants, there is cleaning in which silicon is immersed in oxygenated water mixed with hydrochloric acid to remove the metal contaminants by a chemical reaction, or the like.
As to the etching, there are dry etching using a gas and wet etching using a solution. The wet etching using a solution is used for etching of the entire of a wide surface or for a case where a processed dimension is relatively large in semiconductor micro working.
As described above, in the semiconductor techniques, a wet process is often used in cleaning, etching, and the like in the present circumstances. In this wet process, a water mark is a serious problem. The water mark is such a phenomenon that when a substrate having a silicon surface is dried after wet cleaning or etching, a waterdrop adheres to the substrate surface during the period in which the state of the substrate is changed from a wet state to a dry state, and although the adherent waterdrop evaporate by drying, a mark of the waterdrop remains after the waterdrop disappears.
A suitable Japanese word for the water mark is not established for skilled persons in the art. Various expressions such as a water trace, waterdrop trace, or stain of water are used, and in the present specification, we uses the term of water mark. A definite technical interpretation is not established for what the water mark is, and there is no interpretation beyond the level of a hypothesis.
The following three elements are indispensable for the phenomenon in which the water mark is formed. They are (1) silicon, (2) oxygen, and (3) water. If even one of them is lacking, the water mark is not formed. FIG. 3 shows a commonly accepted theory (for example, Semiconductor World (monthly) 1996. 3, pages 92-94). In step a), oxygen in a dry atmosphere is dissolved in a waterdrop (H.sub.2 O pure water) adherent to the surface of silicon. In step b), the dissolved oxygen is diffused into the interface between the silicon surface and the waterdrop. In step c), an oxide is formed on the silicon surface. In step d), the formed oxide dissolves into silicic acid (presumed to be H.sub.2 SiO.sub.3). In step e), silicic acid is diffused in the liquid and is dissociated, and then it is further diffused. After the waterdrop is dried, the silicon oxide remains on the silicon surface, and this is regarded as the water mark.
If the water mark is once formed, it is extremely difficult to remove it, and substantially it is impossible to remove it. Thus, the water mark functions as a mask at subsequent etching of silicon, so that etching can not be made because of the water mark for a portion where etching is desired, or only partial etching can be made. As a result, silicon at the portion remains unetched.
The size and the number of water marks are greatly changed by conditions at the formation. The size is about 1 .mu.m.phi. to 60 .mu.m.phi., and the number is from several to one thousand or more on a substrate of 5 inches .phi. or 5 inches square. In some case, several water marks of several hundred .mu.m.phi. are formed.
Since it is difficult to remove the water mark when it is once formed, it is important how to prevent the formation. Measures for preventing the formation of the water mark are summarized in the following two measures:
(1) To make the three elements of silicon, oxygen, and water become incomplete.
(2) Not to give a time for reaction (to make a time from water washing to drying as short as possible).
The measure (2) means instantaneous drying, and actually, there is a spin drying method in which a substrate is rotated to dry the substrate by using an air flow and centrifugal force. However, in this method, it is impossible to dry the substrate for such a short time that the water mark is not formed. Thus, the spin drying method is unsuitable for drying in the state where the three elements in the above measure (1) are complete.
As a method of realizing quick drying and eliminating water, there is an IPA (also called isopropyl alcohol, propyl alcohol, or propanol) vapor drying method. In this method, the IPA is heated to produce a vapor. When a substrate is placed in the IPA vapor filling a vessel in a drying apparatus, the vapor of IPA condenses on the substrate, and is replaced with water on the substrate in a short time. According to this IPA vapor drying method, since water and IPA are replaced with each other in a short time, water can be removed from the three elements of the measure (1), and short time drying of the measure (2) can be achieved at the same time. Thus, it is possible to prevent the formation of a water mark at a considerably high probability. Thus, the method is applied to almost all cases in current steps of using a silicon semiconductor.
As other recent methods, a drying system such as a Marangoni system or IPA direct substitution system is proposed, and such a system is actually used in some cases. In the Marangoni system, a substrate is pulled up slowly from pure water into the atmosphere of IPA and nitrogen, and the surface tension of pure water at that time is used. Similarly to the IPA direct substitution system, it can make the water mark zero in principle.
As a method of removing oxygen from the three elements for the formation of a water mark, there are proposed a method of drying a substrate in a nitrogen atmosphere in a closed system, a low pressure drying method of drying a substrate in a low pressure state, and the like. However, it is difficult to completely remove oxygen even in a carrier system in which a treatment is carried out in a state where a substrate is put in a normal carrier, or even in a closed space in a carrierless system (or single wafer processing type) in which only a substrate is processed without enclosing the substrate in a carrier, and it is also impossible to replace oxygen with other gas such as nitrogen in a short time. Also in the low pressure drying method, water and oxygen exist until a low pressure is achieved, so that it is very difficult to prevent the occurrence of a water mark also in this method.
The above described conventional methods of preventing the occurrence of a water mark are such that oxygen or water is removed from the three elements of silicon, oxygen and water for the occurrence of a water mark, or drying is carried out as quickly as possible. There is also a method of removing silicon from the three elements. When a natural oxide film exists on a silicon surface, the water mark is not formed (see the foregoing Semiconductor World 1996. 3, pages 92-94). However, in this method, it is important how to control a natural oxide film to make thin. An uncontrolled thick natural oxide film causes deterioration of a device. Moreover, the method can not be used in a step in which a trace of natural oxide film is not permitted.
As a method of forming a thin oxide film, there is known a chemical oxidation film producing method disclosed in "Ultra Clean ULSI Technique, Tadahiro Ohmi, Baifukan Co., page 214." This chemical oxidation film producing method is applied prior to a step of forming a gate oxide film on a silicon surface, which is especially important when an insulated gate field effect transistor is formed on silicon. In the formation of the gate insulating film, silicon oxide is formed on the silicon surface by using a thermal oxidizing method based on a dry oxidizing technique. Since a channel for flowing carriers is formed in the interface between the silicon oxide and silicon, an oxide film is formed to prevent the silicon surface from being polluted during the transfer to an oxidizing apparatus after cleaning the silicon surface.
Silicon on which the chemical oxidation film was formed is transferred to a thermal oxidizing step as it is, and a thermal oxidation film is formed on the chemical oxidation film. The chemical oxidation film becomes a part of the gate oxide film as it is. Accordingly, the step of forming the chemical oxidation film becomes complicated.
Table 1 shows the manufacturing procedure for forming the chemical oxidation film. The chemical oxidation film is formed through 5 wet steps and 1 dry step, that is, through 6 steps in total.
TABLE 1 Treatment Used Treatment liquid Composition temperature time SPM H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 = 4:1 120.degree. C. 5 minutes O.sub.3 /H.sub.2 O Ultra pure water + O.sub.3 (2 ppm) Room 20 minutes temperature hot H.sub.2 O.sub.2 30% H.sub.2 O.sub.2 80-90.degree. C. 10 minutes APM NH.sub.4 OH:H.sub.2 O.sub.2 :H.sub.2 O = 0.05:1:5 80-90.degree. C. 10 minutes HPM HCl:H.sub.2 O.sub.2 :H.sub.2 O = 1:1:6 80-90.degree. C. 10 minutes Low Dry O.sub.2 300.degree. C. 20 minutes temperature oxidation
When the chemical oxidation film is formed on silicon after a cleaning step, it is possible to prevent the occurrence of a water mark. Since the chemical oxidation film is a precisely controlled oxide film, the film thickness is also controlled and few impurities are also included in the chemical oxidation film, unlike a natural oxide film.
However, when the step of forming this chemical oxidation film is introduced into all of the normal cleaning steps of silicon, the number of steps is greatly increased.
The present inventors paid attention to silicon among the three elements which should not be made complete in order to prevent the occurrence of a water mark. Since the steps of the method using a chemical oxidation film are complicated, the inventors considered forming a controlled oxide film by a simpler means. In general, a substrate is cleaned or etched, and is rinsed by pure water, and then drying is carried out. The inventors have found that a controlled thin oxide film can be formed on a silicon surface during the rinse or after the rinse before drying, and the present invention has been achieved.